1. Field of the Invention
The present invention relates to an apparatus and a method for controlling interference between base stations in a wireless communication system of a hierarchical cell structure. More particularly, the present invention relates to an apparatus and a method for allocating a frequency band to minimize interference exerted by a picocell on a macrocell when the picocell exists in coverage of the macrocell.
2. Description of the Related Art
Recently, a picocell concept is suggested to address a propagation shadow area caused by movement of a terminal between base stations or poor channel state between the base station and the terminal and to provide a high-level data service in a cellular wireless communication system. The picocell covers a smaller area than a macrocell, and a plurality of picocells can reside in the cell coverage of one macrocell.
The macrocell and the picocell can use different frequency bands or the same frequency band. When the macrocell and the picocell use different frequency bands, interference is caused by only a signal out of band emission in each cell. Accordingly, the interference can be addressed by operating the picocell with low power. However, efficiency of the frequency band is low because an additional frequency band is needed. In contrast, when the macrocell and the picocell use the same frequency band, there is no need to use the additional frequency band but considerable interference occurs between the macrocell and the picocell.
As such, various schemes are suggested to control the interference between the cells. An example of one scheme is a Fractional Frequency Reuse (FFR) scheme. The FFR scheme maintains maximum cell efficiency and enhances the efficiency of the cell boundary. For example, as illustrated in FIG. 1, the FFR scheme divides the cell coverage to an inner region and an outer region, sets a frequency reuse factor of the inner cell region to 1, sets the frequency reuse factor of the outer cell region to 3, and thus blocks simultaneous use of the frequency band for the cell outer region. That is, the FFR scheme divides the frequency band for the cell outer region to three partial bands and allocates a user positioned in the cell outer region the partial band not overlapping with the frequency band between neighbor cells.
However, the FFR scheme is suitable for a horizontal cell layout where similar-size cells horizontally reside in succession, and it is difficult to apply the FFR scheme to a hierarchical cell structure including one or more picocells within the coverage of one macrocell.